1. Field of the Invention
The present invention relates to a gel sheet used for cosmetics, pharmaceuticals and the other applications, particularly to a cosmetic sheet or percutaneous absorption preparation to be applied to the skin.
2. Description of the Related Art
Gel sheets mainly comprising agar or the like are conventionally known. Since the agar as a gel forming component forms a network structure in water, such gel sheets can be used for the purpose of moisture retention. The gel sheets made of agar however do not have a sufficient strength and in addition, there is a difficulty in incorporating an oily pharmaceutically active ingredient therein.
Gel sheets using an acrylic polymer gel are, on the other hand, not free from the fear of toxicity of the residual monomer and those with less irritation to the skin are requested. Many gel sheets need a crosslinking agent, the use of which complicates their preparation process. It is also impossible to incorporate a pharmaceutically active oily ingredient in such gels freely.
Many of cosmetic sheets to be applied to the skin such as face or hands contain therein a moisturizing agent and a polymer for supporting the moisturizing agent therewith, or are used after applying components containing a moisturizing agent to the sheet. For example, a cosmetic moisturizing sheet containing a moisturizing agent such as hyaluronic acid or collagen supported on a fiber sheet is disclosed in Japanese Patent Application Unexamined Publication No. 9-238738/1997. A cosmetic pack comprising chitosan and a medium paste, and a collagen-free moisturizing mask obtained by crosslinking a cationic biopolymer are disclosed in Japanese Patent Unexamined Application Nos. 6-48917/1994 and 2001-502678, respectively.
The above-described gel sheet is typically a paper sheet impregnated with a moisturizing agent. It does not have a sufficient water retention capacity and in addition, the paper sheet causes a sense of discomfort upon use. Gel polymers have improved adhesion to the skin, but are not sufficient. Another drawback of them is a complicated preparation process owing to the use of a crosslinking agent.
As a countermeasure against the above-described problems, a gel sheet of cellulose ether having a low molar substitution is disclosed in Japanese Patent Application Unexamined Publication No. 6-48197/1994. The gel sheet is free from the problem of a monomer which occurs upon use of an acrylic polymer, can be prepared without extra effort because a crosslinking agent is not necessary. The gel sheet has a sufficient water retaining capacity, and does not cause any sense of discomfort upon use. When such a sheet is applied to the body, however, it peels off the body before a drug such as a moisturizing agent, an anti-wrinkle agent, an anti-spot agent or a whitening cosmetic with which the sheet has been impregnated starts exhibition of its effect (about 20 minutes). It cannot keep its adhesion for at least 60 minutes, time at least necessary for the effective delivery of the chemical substance. Thus, the gel sheet has a drawback in skin adhesion. The above-described hydrous sheet of low-substituted cellulose ether can be prepared by casting an alkaline solution of low-substituted cellulose ether onto a supporting plate, neutralizing and coagulating the sheet with an acid and then washing off, from the sheet, the salt formed by neutralization. In order to produce a sheet containing a pharmaceutically active ingredient, the pharmaceutically active ingredient is added in advance upon preparation of an alkaline solution, or a sheet formed is dipped in a solution containing the pharmaceutically active ingredient. However, depending on a type of pharmaceutically active ingredient, production of sheet containing the pharmaceutically active ingredient has problems, for example, that the ingredient is decomposed in the alkaline solution, it does not dissolve in the solution smoothly, or a sufficient amount of the ingredient cannot be supported in the sheet even if the sheet is dipped in an aqueous solution of the ingredient. Moreover, an oily substance immiscible with water cannot be incorporated.
There is a process of preparing a hydrous gel sheet having improved skin adhesion, which comprises casting, on a supporting plate, a mixture of an alkaline solution of low-substituted cellulose ether and water-soluble and alkali-soluble cellulose ether having a substitution degree of 1.1 to 1.4, neutralizing and coagulating a sheet of the mixture on the plate with an acid, and washing off the salt in the sheet. By this process, however, it is impossible to prepare a hydrous sheet having a high content of water-soluble cellulose ether because a portion of the water-soluble cellulose ether leaks out during the washing step for removing the neutralization salt, leading to a reduction in the strength of the hydrous sheet. The higher the content of the water-soluble cellulose ether is, the better the skin adhesion of the sheet is. The sheet obtained by the above-described method therefore does not have sufficient skin adhesion.
As percutaneous absorption preparations, a nicotine-containing film preparation is disclosed in Japanese Patent Application Unexamined Publication No. 2003-95947. A skin adhesive film preparation obtained by incorporating a water-soluble cellulose lower-alkyl ether in a fibroblast growth factor is disclosed in Japanese Patent Application Unexamined Publication No. 7-82171/1995. A membrane-attached adhesive multilayer film preparation comprising a water-soluble polymer layer containing a drug and an adhesive layer containing an adhesive substance is disclosed in Japanese Patent Application Unexamined Publication No. 9-235220/1997. Any of them does not relate to a hydrous sheet containing low-substituted cellulose ether.